The term “conveyor chain” in the present context means an endless-loop conveyor device analogous to a conveyor belt, but with the difference that a conveyor chain is comprised of a multitude of rigid segments or links which are connected to each other in a closed loop wherein each link is articulately hinged to a following link and a preceding link. The segments can either be all identical to each other, or a group of dissimilar segments can identically repeat itself around the conveyor chain. The individual segment or group of segments that identically repeats itself is referred to herein as a module or a modular segment and, consequently, the conveyor chain is referred to as a modular conveyor chain.
The radiation transmittance of the endless-loop conveyor comes into play in inspection systems whose geometric arrangement is such that at least part of the scanner radiation passes not only through the products under inspection and the air space surrounding them, but also traverses the endless-loop conveyor. This kind of inspection system is used for example for the detection of foreign bodies in bottled or canned food and beverage products. Of particular concern are metal and glass fragments in liquid products. Due to their higher density relative to the liquid, such foreign bodies will collect at the bottom of the container. Furthermore, if the container has a domed bottom, the foreign bodies will tend to settle at the perimeter where the bottom meets the sidewall of the container. It is therefore very important for the radiographic scanner system to be configured and arranged in relation to the endless-loop conveyor in such a way that the entire inside bottom surface of each container is covered by the scan. Consequently, it is necessary to use a scanner arrangement where at least part of the radiation passes through the bottom of the container and therefore also through the area of the endless-loop conveyor on which the container or any other object to be inspected is positioned.
In a typical arrangement, the rays used for the inspection may for example originate from a source located above and to the side of the conveyor path, enter the container at an oblique angle through the sidewall, exit through the container bottom and pass through the conveyor, to be received by a camera system which is connected to an image-processing system. Alternatively, for example if objects are inspected that are neither bottled nor canned, the radiation source can be arranged vertically above, and the radiation detector vertically below, the conveyor.
If the radiographic inspection system is an X-ray system, the rays can be received for example by an X-ray image intensifier and camera, or by an X-ray line array sensor which, in response, sends a signal to the image processing system. Typically, the imaging radiation originates as a fan-shaped planar bundle of rays from a localized source, i.e. a spot-sized radiation source and is received by a linear array of photodiodes that are collectively referred to as a detector, wherein the fan-shaped radiation bundle and the linear array of photodiodes lie in a common plane, also referred to as the scanning plane, which runs substantially orthogonal to the travel direction of the conveyor carrying the articles to be inspected. While the articles under inspection move through the scanning plane, the linear array of photodiodes is triggered by a continuous sequence of discrete pulses, and the pulse frequency is coordinated with the speed of the conveyor so that the sequence of signals received by the detector array can be translated into a pattern of raster dots with different brightness values expressed for example in terms of a brightness scale from zero to 255, representing a transparent shadow image of the material bodies between the radiation source and the radiation detector. If a scanned article contains foreign objects such as metal fragments, which have a lower transmittance to the scanning rays than the scanned article, the radiographic image will show such foreign objects as darker areas within the transparent shadow image of the scanned article.
At the present state of the art, endless-loop conveyors that are used as transport devices in radiographic inspection systems are in most cases polymer fabric belts. This type of conveyor has the advantage that the quality of the X-ray image is least affected by it, due to the constant thickness and the uniformity of the belt. However, there are a number of strong arguments against polymer fabric belts and in favor of modular conveyor chains, specifically:                There is strong resistance to the use of fabric belts particularly in the bottling and canning industry, because they are easily damaged and wear out rapidly. In comparison, conveyor chains consisting of rigid plastic elements (typically of acetal resin or polypropylene) that are linked together in an endless loop are much stronger and less easily damaged by hard metal or glass containers.        Conveyor chains are better suited for heavy-weight articles such as blocks of cheese, as it is possible to drive the conveyor chain with sprockets that directly engage the chain profile.        The segments of a conveyor chain can be hinged together in such a way that the chain has a unilateral flexibility to loop around the drive sprockets while being rigid against bending in the opposite direction. This latter property eliminates the need for guiding mechanisms which can be unreliable in continuous-duty applications.        Conveyor chains are easier to replace or repair than belts, because the chain can be opened by removing one of the hinge pins by which the modular segments of the chain are linked together.        Conveyor chains can be designed to be self-tracking and to run flush with the sides of the conveyor support structure. This last characteristic is important, because it allows products to be easily transferred sideways between laterally adjacent conveyors.        
Nevertheless, the use of customary chain conveyors with plastic chain links is problematic in radiographic inspection systems, because the chain links can interfere with the X-ray image. Until now, if one wished to X-ray a product moving on a conveyor chain, the resultant image was degraded by the variations in the transmittance of the conveyor chain superimposed on the product, for example due to hinges or other connections between the chain segments or by profile features designed to stiffen the chain segments. If this problem of image interference can be solved, the benefits of modular conveyor chains as listed above can be applied to radiographic inspection systems.
In US published application 2012/0128133 A1, which is owned by the same assignee as the present invention, the problem of transmittance variations is solved through a conveyor chain in which the chain segments are configured in essence as rigid plates of uniform thickness and density extending over the width of the conveyor chain, wherein the segments overlap each other to present themselves to the scanner radiation as a substantially gapless band of uniform transmittance and wherein the connectors or hinges which link the segments together (and which have a lower transmittance than the flat areas of the segments) are located outside the band that is traversed by the scanner radiation. Thus, the connections between the segments are preferably located in the two lateral border areas of the conveyor chain.
In a conveyor chain according to the foregoing concept, the absence of hinges or any other stiffening features in the central homogeneous band area reduces the rigidity of the chain segments in regard to transverse bending and therefore limits the conveyor width that can be realized in a practical design.
Another solution is offered in unpublished European patent application published as EP 2711694 A1 which is likewise owned by the assignee of the present invention and whose entire content is hereby incorporated by reference in the present disclosure. In short, a method of operating a radiographic inspection system is described which is specifically designed for a system in which a conveyor chain with identical modular chain segments is transporting the articles under inspection.
The method of European patent application EP 2711694 A1 encompasses two operating modes of the radiographic inspection system. In a first mode, referred to as calibration mode, an image of one modular segment of the empty conveyor chain is recorded and stored in the form of digital pixel data, referred to as calibration data. The reason why the calibration data are recorded only for one modular segment is that the set of calibration data repeats itself identically for each modular segment of the chain. In a second mode, referred to as inspection mode, an image of the articles under inspection with the background of the conveyor chain is recorded in the form of digital pixel data, referred to as raw image data. Immediately, i.e. after each row of pixels has been recorded, the raw image data are arithmetically processed into a clear output image by cancelling out the interfering background of the conveyor chain with the help of the calibration data.
In order to be able to geometrically correlate the raw image data collected in the inspection mode to the calibration data collected and stored in the calibration mode, every raster dot of the radiographic image needs to be registered in terms of x/y-coordinates relative to the modular segment, so that each pixel value can be stored together with its x/y address.
The registration coordinate x in the transverse direction of the modular segment (parallel to the hinge pin) can simply be based on the array position of the photodiode associated with that location within the radiographic image.
For the registration coordinate y in the longitudinal direction of the chain (i.e. the direction of conveyor movement) European patent application EP 2711694 A1 proposes the concept of a physical registration feature that is either part of the chain or moves together with the chain. In a preferred embodiment, the registration feature is realized as a ramp-shaped lateral border portion formed on the modular segment. In operation, the ramp-shaped border intercepts a marginal sector of the fan-shaped radiation bundle, so that the image received by the sensor array will be representative of the ramp height, from which the longitudinal registration coordinate y can be directly calculated by the processor.
In the foregoing method and apparatus according to European patent application EP 2711694 A1, the physical registration feature as exemplified by the ramp is a feature that is not normally present in a conveyor chain and is added for the sole purpose of providing a registration reference in the longitudinal direction y of the conveyor chain. It occurred to the inventors that, as an alternative to adding a physical feature to the conveyor chain, the raw image data collected in the inspection mode could also be geometrically correlated to the calibration data by using known techniques of image-matching and image interpolation to register an inspection image directly against the stored background image without the help of a specifically dedicated physical registration feature.
In view of the strong potential seen in the techniques of image matching and interpolation as a solution to the problem of accurately registering an inspection image against a calibration image in the process of radiographic product inspection, the present invention has the objective to provide a viable alternative to the use of a specifically dedicated physical registration feature for geometrically matching the raw images of inspected articles to the background image of the modular conveyor chain.